The present invention relates to starting aids for high pressure sodium vapor discharge lamps, and more particularly, such a starting aid having simplified structure.
High pressure sodium (HPS) discharge lamps comprise a discharge vessel containing a pair of discharge electrodes, and a sodium amalgam which is partially vaporized and ionized during lamp operation to produce light. Some types of HPS lamps also have an inert buffer gas within the discharge vessel for influencing the internal pressure and temperature of the atmosphere within the discharge vessel during operation.
The inert buffer gas makes the lamp difficult to start before the discharge electrodes are heated if the buffer gas fill pressure is above a certain value range, typically 50 torr. In order to facilitate starting it is conventional to provide a starting aid which promotes ionization within the buffer gas, prior to the onset of arc discharge, in order to lower the breakdown voltage within the discharge vessel.
These starting aids have various constructions such as a length of wire wrapped around the outside surface of the discharge vessel as shown, for example, in U.S. Pat. No. 4,037,129, and British Patent Nos. 1,493,270, 1,340,551 and 1,569,305. Another type of starting aid is comprised of a rod-like conductor extending along a substantial portion of the length of the discharge vessel, as shown in U.S. Pat. Nos. 4,277,725, 4,328,445, 4,498,030 and 4,521,716. Still another construction, comprising a bimetallic element which contacts the discharge vessel when the lamp is cold and springs away when the lamp is heated is disclosed in U.S. Pat. No. 3,872,340.
All of the starting aids discussed above comprise several structural elements which must be assembled as part of the lamp during lamp fabrication. This makes the lamp more costly. In addition, in some designs parts of the starting aids are made of niobium, and entail niobium welds. Niobium is a difficult metal to weld, and these welds are susceptible to failure from environmental causes such as vibration. If the weld fails, the starting aid may become ineffective and the lamp will be impossible to start.
It is accordingly an object of the invention to provide an effective starting aid for HPS discharge lamps which has a simplified structure and fewer parts in order to improve reliability and decrease cost.